1. Field of the Invention
The present invention relates to a highly directional microphone that includes an acoustic tube, and specifically, a highly directional microphone of which the directivity can be fine-tuned by a user.
2. Related Background Art
In a highly directional microphone that includes an acoustic tube, a microphone unit is disposed inside one end in a longitudinal direction or middle of the acoustic tube. Sound waves from directions other than a target direction, that is, a front end of the acoustic tube, interfere with and are cancelled by sound waves from openings on the side wall of the acoustic tube due to a time lag therebetween. The highly directional microphone, thus, has high sensitivity to the sound waves from the front end of the acoustic tube to obtain narrow directivity. Directivity of the highly directional microphone, therefore, depends on the wavelength of sound and the length of the acoustic tube. A long acoustic tube exhibits narrow directivity over a wide frequency range up to low frequency, while a short acoustic tube exhibits narrow directivity only in a high frequency region.
In general, a highly directional microphone including an acoustic tube and a unidirectional condenser microphone unit which are combined with each other, is designed to operate in a unidirectional mode at a frequency band equal to or lower than a band in which the acoustic tube exhibits narrow directivity.
An example of the highly directional microphone including acoustic tube and a unidirectional condenser microphone unit which are combined with each other is disclosed in Japanese Unexamined Patent Application Publication 2000-083292.
In general, a polar pattern at a low frequency band of the highly directional microphone including a combination of the condenser microphone unit and acoustic tube is designed to be hypercardioid for reducing the sound waves from the side direction. If a noise source is present at 180-degree direction, that is, at the rear end of the acoustic tube, the sound waves of an extremely low frequency band are disadvantageously picked up. Within the frequency band in which the highly directional microphone unidirectionally operates, it is preferable to adjust the angle in order to avoid a reduction in the sensitivity in response to the direction of the noise source, that is, it is preferable to adjust the directivity.
A possible measure to adjust the directivity of the highly directional microphone is adjustment of the acoustic resistance of the microphone unit. FIG. 13 illustrates an exemplary conventional highly directional microphone of which the directivity can be adjusted by adjusting the acoustic resistance of the microphone unit incorporated in the acoustic tube. FIG. 13 illustrates an elongated cylindrical acoustic tube 110 one end of which is connected to a tubular microphone unit holder 120. A microphone unit 130 is disposed inside the tubular microphone unit holder 120. Hereinafter, the end of the acoustic tube 110 at which the microphone unit 130 is disposed is referred to as a rear end and the opposite end thereof as a front end. In this example, the microphone unit 130 is a condenser microphone unit and, as is well known, includes a diaphragm composed of a thin film and a fixed electrode that faces the diaphragm with a slight gap therebetween. The microphone unit 130 itself has unidirectional directivity and includes the diaphragm that is disposed so as to face the front end of an acoustic tube 110. The diaphragm and the fixed electrode constitute the condenser. Vibration of the diaphragm receiving the sound waves varies the capacitance of the condenser, and the variable capacitance is output as a change in electric signal. A front cap 160 is attached to the front end of the acoustic tube 110.
Slits (not shown) are formed on the peripheral surface of and parallel to the central axis of the acoustic tube 110. The sound waves from directions other than the target direction, that is, other than the front-end direction of the acoustic tube 110 enter the acoustic tube 110 through the slits and the front end of the acoustic tube 110. The sound waves that enter the acoustic tube 110 through the slits and the sound waves that enter the acoustic tube 110 through the front end thereof interfere with and cancelled by each other inside the acoustic tube 110 because they enter the acoustic tube 110 at a certain time lag. Accordingly, the sound pressure that reaches the microphone unit 130 decreases. In contrast, the sound pressure of the sound waves from the front end direction of the acoustic tube 110 does not decrease. Thus, the sound waves from the front end direction are dominantly electro-acoustically converted. This achieves narrow directivity.
As explained above, in the highly directional microphone including a combination of the acoustic tube and the highly directional microphone, the acoustic resistance is adjusted for adjustment of the directivity. The conventional narrow directivity microphone in FIG. 13 includes an acoustic resistive material 133 that is disposed behind the diaphragm of the microphone unit 130 and determines the acoustic resistance of the rear acoustic terminal, and an adjustable nut 135 which adjusts the acoustic resistance by adjusting the urging force of the acoustic resistive material 133. The acoustic resistance of the acoustic resistive material 133 varies with the extent of tightening of the adjustable nut 135 to adjust the directivity.
As is shown by the conventional highly directional microphone in FIG. 13, the directivity of the highly directional microphone including a combination of the acoustic tube and a highly directional microphone can be adjusted. The adjustment of the directivity of the conventional highly directional microphone, however, requires skillful adjustment of the adjustable nut 135 of the microphone unit 130 disposed in the acoustic tube 110 or the tubular microphone unit holder 120. Since the microphone unit 130 must be directly adjusted, improper adjustment creates various problems, such as damage of the diaphragm and an increase in noise due to decreased insulation. As matters now stand, therefore, it is difficult to adjust the directivity by a user without asking a manufacture to adjust the directivity.